Motor Insulation resistance was bad 1

[jonos]

Dear All,

Please, I need some advice,
I have a problem with my motor. The motor with 380 V, 3 phase, 50 Hz, 2 pole, 150kW suddenly shutdown after running around 2 hours. The 50G relay (groundfault relay) was actived. I measured insulation resistance by 500VDC and winding resistance , result :

R phase 2.55 Mohm/ 0.0135 ohm
S phase 2.50 Mohm/ 0.0136 ohm
T phase 2.60 Mohm/ 0.0135 ohm

Based on this, I started motor again until 4 hours and I measured current (ampere) and voltage each phase were normal and vibration was normal ( there was nothing electrical problem ). Vibration at 2H (maximum value) overall below 1.5 mm/s RMS. I take with Fmax 200 Hz and resolution 6400 lines.

I stopped motor and I measured insulation resistance was increased each phase around 50 Mohm. I conducted heating up the motor until insulation resistance reach 150 Mohm.

After 3 days, I checked insulation resistance and I got insulation resistance were decrease (each phase) become 3.50 Mohm

Would you like to explain me:

What is problem with my motor?
Why insulation resistance decrease in 3 days?

Please, give me some advice for this condition.


Regards,
pjono

 

[torslum]

You did not mention anything about age, condition or environment. My first thought is that the motor could be dirty or situated in moist air et.c? Particularly since the insulation resistance seems to increase while you've run it for a while, and the heat could have removed some moist or whatever. Then after three days, it is all back again?

Maybe opening the motor to do cleaning and service would help. You could open one cover (depending on the IP-degree, though) and use your flashlight to look if it sounds reasonable.

Does this make sense to your application?

 

[jonos]

The motor age around 9 years, environment was dried (hazardous area), condition around the motor was good.

Yes, after 3 days, IR was become low again.

Whait is "use flashlight" mean, Sir?
Would you like to explain about that, please?

Thanks

 

[stardelta]

Sounds like a condensation build up inside the motor is causing your problem. Is it located outdoors? If the motor is in a hazardous area it will be an Ex or explosion proof design. This type of motor is prone to condensation build up as the design does not allow the winding to vent very well. Heating the winding will displace the excess moisture from the winding but as the motor is still aseambled and the casing is almost gas-tight the moisture has nowhere to go and remains inside the casing where over a period of time it will find its way back into the winding. The motor needs to be overhauled and the winding cleaned and stoved to restore the IR to an acceptable level, not a difficult job for a competent repair company but note not all repair companies are qualified or experienced in Ex repairs and you should obtain proof of competance before allowing them to commence work on your equipment. I would also advise that when you check the IR to earth, you also check the IR petween phases, it should be of a similar value for a sevicable machine.

 

[jonos]

Stardelta,

Yes, motor in outdoors.
Thank you for your explanation.
Any way, thank you for all to give me some advice.

Regards
jonos

 

[electricpete]

I forgot to ask - did you determinate the motor from the cable during your resistance checks?

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